You have XP? So what's the problem then?

If the files are installed on a NTFS volume, you can assign/restrict any access rights (including the execute right) to any file (including the Yahoo Messenger and/or AIM executables) - it's a feature of the OS since Windows NT 3.1...
To set it, just right-click the exe file(s) in Explorer, click Properties, and see the Security tab.
Cheers
Vlk